A. Field of the Invention
The present invention relates to an ultrasound diagnosis apparatus for displaying 3-dimensional (3D) images by performing a 3D scan over a target region by using a 2-dimensional (2D) array ultrasound probe. More particularly, the present invention relates to a parallel simultaneous reception-type ultrasound diagnosis apparatus that can acquire 3D image data (hereinafter, “volume data”) with an increased acquisition rate of 3D ultrasound image data (volume rate) by eliminating heterogeneous sensitivities among parallel simultaneous receiving beams.
B. Background of the Invention
An ultrasound image diagnosis apparatus transmits and receives ultrasound through a plurality of ultrasound transducers installed in an ultrasound probe to and from a target in an object in a plurality of directions in order to display the image of the target on a monitor. Since an ultrasound image diagnosis apparatus can easily obtain and display a 2D image or a 3D image in real time by simply touching an ultrasound probe to a patient's body surface, it is widely used as an apparatus for diagnosing the status of a target organ in a patient's body.
In particular, displays of B mode image data acquired by using an ultrasound pulse reflection method and color Doppler image data acquired by using an ultrasound image Doppler method are inevitable for an ultrasound image diagnosis for tissues and blood cells in a living body.
Generally, an electronic scan-type ultrasound diagnosis apparatus uses a plurality of transducers arranged in a line and displays 2D image data in real time by electrically controlling the drives of the plurality of transducers at a high speed. The 2D image data is generated by performing 2D scans over an object. Recent years, it has become possible to utilize an ultrasound diagnosis apparatus that can generates 3D image data at an optional cross-section, such as 2D image data (Multi-Planar Reconstruction (MPR) image data) or volume rendering image data by performing 3D scans and to acquire wide range data of a 3D region of an object in a short time.
To acquire the volume data, there are two kinds of acquiring methods. One is a mechanical data acquisition method that moves or rotates a conventional ultrasound probe including linearly arranged transducers. The other is an electronic data acquisition method that electronically controls driving signals for a plurality of transducers arranged in a 2D array and also electronically controls receiving signals acquired through the 2D array transducers. While a mechanical data acquisition type ultrasound diagnosis apparatus is relatively easy to construct, it needs a lot of time for acquiring volume data. Accordingly, it is very difficult for the mechanical data acquisition type ultrasound diagnosis apparatus to correctly acquire data of quickly moving organs or blood flow. On the contrary, although an ultrasound probe and a main body for an electrically data acquisition type ultrasound diagnosis apparatus requires complex constructions, it can acquire volume data in a short time. In particular, with accompanying a 3D scan synchronized with heart beats (triggered volume scan), it becomes possible to display 3D image data of a quick moving organ, i.e., a heart, as motion pictures.
However, while the triggered volume scan is effective to the diagnosis for a circulation organ, it can not to apply to an object having a severe arrhythmia. Further, since a blood flow measurement under the ultrasound Doppler method requires a plurality of ultrasound transmissions and receptions on the same portion in order to acquire the flood speed data, it becomes difficult to display 3D color Doppler image data in real time.
In order to improve acquisition speed of the volume data, it has been proposed to apply the parallel simultaneous reception method for simultaneously receiving echo signals from a plurality of directions (For instance, Japanese Patent Application Publication 2000-116651). FIG. 9 explains the 4-beams parallel simultaneous reception method by using a 2D array ultrasound probe in which a plurality of transducers are arranged both in the θ (azimuth) direction and the φ (elevation) direction. The 4-beams parallel simultaneous reception is performed by 2 beams parallel simultaneous reception both in the θ (azimuth) direction and the φ (elevation) direction for each transmission beam region (hereinafter, “transmitting acoustic field”). For instance, in the transmitting acoustic field T1 having a transmitting beam center axis (●) C1,2 beams parallel simultaneous receptions in the θ (azimuth) direction and 2 rows parallel simultaneous receptions in the φ (elevation) direction are performed. Thus, 4 reception beam directions (◯) R1, R2, R3, and R4 are received from the transmitting acoustic field T1 as one receiving group.
According to the conventional 4-beams parallel simultaneous reception method, as illustrated in FIG. 9, a plurality of transmitting acoustic fields T1, T2, . . . , Tn is set on a 3D region of an object so as to be located at a prescribed angular distance Δξ0 from each other. Further, in each transmitting acoustic field, for instance T1, each of the parallel simultaneous reception 4 beams, for instance R1, R2, R3, and R4 is set at an equal direction of an angular distance Δξ apart from a beam center axis C1 of the transmitting acoustic field T1 so as to obtain an equal transmission intensity to each of the parallel simultaneous reception 4 beams. Consequently, a transmission/reception sensitivity that is decided by a product of the transmission intensity and the reception sensitivity is also equal to each of the parallel simultaneous reception 4 beams in each transmitting acoustic field. Thus, by performing 3D scans in accordance with the 4-beams parallel simultaneous reception method, 3D ultrasound image data (volume data) can be acquired with a small unevenness of sensitivity among the 4 receiving beams.
However, when the number of the receiving beams for the parallel simultaneous reception is increased to more than five (5) in order to increase an acquisition rate of the volume data (volume rate), it becomes impossible to obtain equal transmission/reception sensitivity by applying the conventional parallel simultaneous reception method. Thus, problems of sensitivity unevenness and sensitivity differences would occur.
FIG. 10 illustrates a case that the number of the receiving beams for the parallel simultaneous reception is increased to eight (8) in the conventional parallel simultaneous reception method. To perform 8 beams parallel simultaneous reception, 4 receiving beams arranged in the θ (azimuth) direction and 2 rows of the 4 receiving beams in the φ (elevation) direction are received as one group of the parallel simultaneous reception beams. Thus, two rows of 4 parallel simultaneous reception beams are set against a transmitting acoustic field. For instance, parallel simultaneous reception 8 beams R11 to R18 are set to a transmitting acoustic field T1 having a center axis C1 as setting 4 beams in the θ (azimuth) direction and 2 rows in the φ (elevation) direction. In this case, as shown in FIG. 10, a first angular distance Δξ1 between the center axis C1 of the transmitting acoustic field T1 and each of the parallel simultaneous reception 4 beams R11, R14, R15, and R18 that are located at a far-off position from the center axis C1 becomes larger than a second angular distance Δξ2 between the center axis C1 of the transmitting acoustic field T1 and each of another parallel simultaneous reception 4 beams R12, R13, R16, and R17 that are located near to the center axis C1 in the transmitting acoustic field T1. Consequently, the transmission/reception sensitivity for each of the parallel simultaneous reception 4 beams R11, R14, R15, and R18 becomes lower than the transmission/reception sensitivity for the parallel simultaneous reception 4 beams R12, R13, R16, and R17 that are located near to the center axis C1 in the transmitting acoustic field T1.
FIG. 11A is a model for illustrating a relationship between the transmitting acoustic field AT1 having a center axis C1 shown in FIG. 10 and each of reception beam regions (acoustic fields) AR11 to AR14 of the parallel simultaneous reception beams R11, R12, R13, and R14 corresponding to the transmitting acoustic field AT1. A beam width of the transmitting acoustic field AT1 is set so as to be larger than each beam width of the receiving acoustic fields AR11 to AR14 in order to acquire sufficient transmission/reception sensitivities for all of the parallel simultaneous reception beams R11 to R14.
FIG. 11B illustrates a transmission intensity distribution BT1 of the transmitting acoustic field AT1 at a cross-section Zx orthogonally crossing the center axis (z axis) shown in FIG. 11A and reception sensitivity distributions BR11 to BR14 for the parallel simultaneous reception 4 beams. FIG. 11C illustrates each of the transmission/reception sensitivity distributions BX11 to BX14 of the parallel simultaneous reception 4 beams R11 to R14 that is respectively calculated as a product of the transmission intensity distribution BT1 shown in FIG. 11B and each of the reception sensitivity distributions BR11 to BR14.
As apparent from the transmission/reception sensitivity distributions shown in FIG. 11C, sensitivity differences occur among the transmission/reception sensitivities BX12, BX13 for the reception beams locating near to the transmitting acoustic field center axis and the transmission/reception sensitivities BX11, BX14 of the reception beams locating far apart from the transmitting acoustic field center axis. Thus, when the number of parallel simultaneous reception beams are increased to 8 by applying the conventional parallel simultaneous reception, transmission/reception sensitivity differences would be generated depending upon the angular distances between the transmitting acoustic field center axis and each of the parallel simultaneous reception beam directions. This causes a non-permissible problem of sensitivity unevenness in the acquired volume data. To reduce such unevenness of the transmission/reception sensitivity, it is intended to expand the beam width of the transmission beam. However, by doing so, the transmission intensity distribution BT1 is also reduced. Consequently, the transmission/reception sensitivities for each of the reception beams are deteriorated. This causes another problem of a deterioration of the S/N ratio for the volume data.